[StepSecurity] ci: Harden GitHub Actions (#17767)

## Summary

This pull request is created by
[StepSecurity](https://app.stepsecurity.io/securerepo) at the request of
@lukeheath. Please merge the Pull Request to incorporate the requested
changes. Please tag @lukeheath on your message if you have any questions
related to the PR.
## Security Fixes

### Least Privileged GitHub Actions Token Permissions

The GITHUB_TOKEN is an automatically generated secret to make
authenticated calls to the GitHub API. GitHub recommends setting minimum
token permissions for the GITHUB_TOKEN.

- [GitHub Security
Guide](https://docs.github.com/en/actions/security-guides/automatic-token-authentication#using-the-github_token-in-a-workflow)
- [The Open Source Security Foundation (OpenSSF) Security
Guide](https://github.com/ossf/scorecard/blob/main/docs/checks.md#token-permissions)
### Pinned Dependencies

GitHub Action tags and Docker tags are mutable. This poses a security
risk. GitHub's Security Hardening guide recommends pinning actions to
full length commit.

- [GitHub Security
Guide](https://docs.github.com/en/actions/security-guides/security-hardening-for-github-actions#using-third-party-actions)
- [The Open Source Security Foundation (OpenSSF) Security
Guide](https://github.com/ossf/scorecard/blob/main/docs/checks.md#pinned-dependencies)


## Feedback
For bug reports, feature requests, and general feedback; please email
support@stepsecurity.io. To create such PRs, please visit
https://app.stepsecurity.io/securerepo.


Signed-off-by: StepSecurity Bot <bot@stepsecurity.io>

Signed-off-by: StepSecurity Bot <bot@stepsecurity.io>

.github/workflows/deploy-vulnerability-dashboard.yml

@@ -6,8 +6,13 @@ on:
     paths:
       - 'ee/vulnerability-dashboard/**'
 
+permissions:
+  contents: read
+
 jobs:
   build:
+    permissions:
+      contents: write  # for Git to git push
     if: ${{ github.repository == 'fleetdm/fleet' }}
 
     runs-on: ubuntu-latest